In addition to the long familiar metallic superconductive materials such as NbTi or Nb3Sn, which have very low transition temperatures Tc and are therefore also referred to as low Tc superconductive materials or LTS materials, metal oxide superconductive materials with transition temperatures of more than 77 K have been known since 1987. The latter materials are also referred to as high Tc superconductive materials or HTS materials, and in principle enable a cooling technology using liquid nitrogen (LN2).
The attempt is also being made to produce superconductive windings with conductors by using such HTS materials. It has been found, however, that conductors known to date only possess a relatively low current carrying capacity in magnetic fields with inductions in the tesla range. This frequently imposes the necessity that the conductors of such windings, in spite of the intrinsically high transition temperatures of the materials used, must nevertheless be kept at a temperature level which lies below 77 K, for example between 10 and 50 K, in order thus to be able to carry significant currents in the presence of field strengths of a few tesla. Such a temperature level lies substantially higher than 4.2 K, the boiling temperature of liquid helium (LHe), with which known metallic superconductive materials such as Nb3Sn or NbTi are cooled.
Refrigeration units in the form of cryocoolers with a closed He compressed gas circuit are therefore preferably used in the said temperature range for cooling windings with HTS conductors. In particular, such cryocoolers are of the Gifford-McMahon or Stirling type or are realized as so-called pulse tube coolers. Such refrigeration units additionally have the advantage that the refrigeration power is available almost at the touch of a button and the user is spared the handling of low-temperature liquids. Where such refrigeration units are used, a superconductive device such as a solenoid coil or a transformer winding is only cooled indirectly by means of thermal conduction to a refrigeration head of a refrigerator (cf. e.g. “Proc. 16th Int. Cryog. Engng. Conf. (ICEC 16)”, Kitakyushu, JP, 20-24.05.1996, Elsevier Science, 1997, pages 1109 to 1129).
A corresponding cooling technique is also provided for a superconductive rotor of an electrical machine which can be taken from U.S. Pat. No. 5,482,919 A. The rotor contains a rotating winding of HTS conductors which can be kept at a desired operating temperature between 30 and 40 K by means of a refrigeration unit designed as a Stirling or Gifford-McMahon or pulse tube cooler. For this purpose, the refrigeration unit contains, in a special embodiment, a co-rotating refrigeration head which is not described further in the specification, the colder side of which is thermally coupled to the winding indirectly by way of thermally conducting elements. Furthermore, the refrigeration unit of the known machine comprises a compressor unit situated outside its rotor which supplies the required working gas to the refrigeration head by way of a rotating coupling, which is not described in detail of a corresponding transfer unit. Additionally, by way of two slip rings, the coupling also supplies a valve drive mechanism of the refrigeration unit, which is integrated into the refrigeration head, with the necessary electrical energy. This concept requires that at least two gas connections must be routed coaxially and at least two electrical slip rings provided in the transfer unit. Additionally, the accessibility of the co-rotating parts of the refrigeration unit and in particular of the valve drive mechanism in the rotor of the machine is hampered since the rotor housing must be opened in the case of maintenance being required. Furthermore, the function of a conventional valve drive mechanism is not assured in the case of rapid rotation such as is present in the case of synchronous motors or generators.